Published on 20-Mar-2024

Basic Principles of Magnetic Particle Testing every NDT personnel should know

Basic Principles of Magnetic Particle Testing every NDT personnel should know

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Magnetic Particle Testing is a widely used non-destructive testing method as it is extremely simple and faster to apply than other methods. Part surface preparation is also not that critical when it is compared to other methods. It uses small magnetic particles, i.e. iron filings and magnetic fields, to discover defects in the components.

The one criteria that need to be fulfilled is that the component which is being inspected should be made of material that can be magnetized, i.e. ferromagnetic material, for example - cobalt, iron, nickel, etc. Now, let's move on to the part that describes the principle of Magnetic Particle Testing in detail. 

What is Magnetic Particle Testing?

Magnetic Particle Testing

Magnetic Particle Testing (MPT), also referred to as magnetic particle inspection (MPI) in the field of NDT, is a crucial method employed for flaw detection in metallic components. This NDT Technique relies on the interaction between magnetic fields and small magnetic particles to identify defects such as cracks and corrosion. Here's a detailed explanation focusing on the specified keywords:

Definition and Purpose of MPT:

MPT is a renowned NDT Method, that serves the purpose of identifying flaws in metallic components without causing damage. It is vital to ensure the reliability and safety of equipment and structures by detecting surface and near-surface defects, including cracks and corrosion.

Learn More About: How to Perform Magnetic Particle Testing

Magnetic Particle Testing Techniques


Magnetic Particle Testing Techniques

Magnetic Particle Testing encompasses various techniques aimed at effectively detecting defects in metallic components without causing damage. Here's a detailed breakdown of the techniques involved:

1. Magnetization Process:

Magnetization is a crucial step in MPT, involving the creation of a magnetic field within the component under examination. This process can be achieved through methods such as Direct Magnetization, where a current is passed through the component, or indirect magnetization, utilizing a separate magnetic source.

Direct magnetization involves applying an electric current directly to the component, inducing a magnetic field within it. Indirect magnetization utilizes external magnetic sources such as yokes or electromagnetic coils to induce a magnetic field in the component.

2. Particle Application:

Once magnetized, the component is coated with magnetic particles, typically iron filings, or ferromagnetic powders. These particles adhere to the surface of the component, forming distinct patterns in response to the magnetic field. The application of particles is usually achieved through methods such as dry powder dusting or wet suspension spraying. Dry powder dusting involves spreading dry magnetic particles over the surface of the component, while wet suspension spraying utilizes a liquid carrier to suspend the particles and apply them evenly.

3. Clustering Process:

Defects in the component disrupt the magnetic field, causing the magnetic particles to cluster around the flaw. This clustering effect creates visible indications, making the defects readily apparent for inspection. By observing the distribution and pattern of clustered particles, NDT Technicians can identify the presence, size, and location of defects accurately.

Magnetization Considerations

Proper magnetization is essential for ensuring the effectiveness of MPT Techniques. Here are key considerations when magnetizing components for MPT:

Magnetic Field Strength:

The strength of the magnetic field applied to the component significantly influences the detection sensitivity.

Adequate magnetic field strength is crucial for ensuring that defects generate sufficient Magnetic Flux Leakage to attract particles.

Direction of Magnetization:

The direction in which the magnetic field is applied plays a crucial role in detecting defects oriented parallel or perpendicular to the magnetic field lines. Proper orientation ensures that defects are effectively highlighted by clustered particles.

Duration of Magnetization:

The duration for which the component remains magnetized affects the inspection time and sensitivity. Longer magnetization durations may improve defect detection sensitivity but can also increase inspection time and complexity. Understanding and implementing proper magnetization techniques and considerations are critical for achieving accurate and reliable results in MPT

By adhering to established techniques and considering factors such as magnetic field strength, direction, and duration of magnetization, NDT Professionals can effectively identify defects and ensure the integrity of metallic components without causing damage.

What are the Basic Principles of MPI?

Magnetic Flux Leakage Field

1. If the center of a bar magnet is broken, it will result in two different bar magnets having their magnetic poles at both ends. But if there's only a crack in between, then at each edge of the crack, the north and south poles will be shaped.

2. When the Magnetic Fields get introduced with the small air gap, which has been developed due to the crack, the magnetic field spreads out because the air is not able to support the magnetic field per unit volume when compared to the magnet. So, when the magnetic field spreads out, it seems to leak out of the material, and hence, this is known as a Magnetic Flux Leakage Field. 

3. Moreover, if the iron particles are drizzled over the bar magnet where it has been cracked, the iron particles will be clustered and attracted to the poles located at the end of the magnet as well as to the poles at the edges of the cracked point. The cluster of magnetic particles is understandable and easier to watch than the existing crack, and therefore, this becomes the ground for Magnetic Particle Inspection


Principles of Magnetic Particle Testing

● Now, the next step that needs to be performed is to magnetize the component while doing a Magnetic Particle Inspection. If any flaws are detected on the surface, a leakage field will be created by the flaws. 

● Therein the components are magnetized, and the iron particles, no matter which forms they are in, i.e. wet or dry suspended form, are applied to the surface of the magnetized portion. 

● Thus, the iron particles will be clustered and attracted to the Magnetic Flux Leakage Fields, which leads to forming a noticeable indication that can be detected by the inspector. 

This particular method detects various elements that include weldments, forging & casting. There are so many industries that opt for Magnetic Particle Testing, like petrochemical, power generation, automotive, aerospace, and steel industries. It can also be utilized for Underwater Inspection, where it can be used for testing underwater pipelines and offshore structures. 

Key Takeaways

  • MPT, also known as MPI, is a vital NDT method used for flaw detection in metallic components without causing damage.
  • Learn about magnetization processes, particle application methods, and defect clustering for effective defect identification.

FAQs

1. What is the application of magnetic particle testing?

A: Magnetic Particle Testing is applied to detect surface and near-surface defects in ferromagnetic materials, commonly used in industries like aerospace, automotive, and manufacturing.

2. What is the working principle of crack detection using magnetic particle testing?

A: The working principle involves magnetizing the component, causing magnetic particles to cluster around cracks, and making them visible under UV light.

3. Which liquid is used in MPT?

A: Fluorescent or non-fluorescent magnetic particle suspensions are commonly used liquids in Magnetic Particle Testing.

4. What are the two methods of testing by magnetic particle inspection?

A: The two methods are Dry Particle Inspection, where dry particles are applied to the component, and wet suspension inspection, where particles are suspended in a liquid carrier and sprayed onto the component.

References

1. WDB Group

2. BINDT



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